It may well be worth checking to see if any of your security features...antivirus and such are being a little over zealous.
Add the CREATE locations to any exclusions / white list you may have in that regard.
That fixed a lot of issues at this end and for a few other people too.
Also maybe check your YUN has the latest linux as well as that also helped quite a bit.
Its not such an easy process but it was worth it and is reasonably well documented in the YUN section and the YUN product part of the site.
I can now upload anywhere on my network or via the USB port.
Note that IP uploads are not yet available in CREATE so you must stick to USB at the present time.
Also avoid USB 3.0 ports as there have been quite a few issues seen with them.